<!doctype html> 
<html> 
<head> 
    <meta charset="utf-8"> 
    <title></title> 
    <style type="text/css">
body>span:only-of-type{
  width: 100px;height: 100px;
  float: left;background: black;
}
</style>
</head>
<body>
<div></div>
<div></div>
<div></div>

<script type="text/javascript">

	var a = document.querySelector('div');
	a.style.cssText = 'background:black;width:200px;height:200px;'
		

</script>
</body>
</html>